The long term objective of this research is to identify the factors, which determine influenza virus host range. Influenza A viruses grow in various animal species, yet most strains are limited in host range. Since viruses from infected lower animals provide a source of strains which can cause pandemic disease in the human population, these host range limitations can be overcome. How this comes about is poorly understood, although there is growing evidence that changes in the viral hemagglutinin (HA) accompany interspecies transfer. The HA has various functions associated with it and one can show that changes in these functions affect selection of these viruses by host cells. This work will identify changes in HA sequence that result in host-dependent changes in the affinity of the HA for cellular receptors. A battery of HA mutants which differ from each other at different sites will be isolated and characterized by determining their receptor binding properties. The sequence of their HA genes, and when appropriate, the structure of the oligosaccharides on the HA protein. Secondly, HA sequence changes which occur when influenza A viruses move from one species of animals to another, and when these viruses are grow in the laboratory will be documented. The sequence of the HA gene of viruses taken directly from patients and from wild birds will be compared with those obtained from the same viruses after growth in cells commonly used to isolate the influenza viruses. These viruses of known HA sequence will then be used to infect ferrets and the sequence determinations will be repeated on virus recovered from the ferrets in order to determine which of the possible mutations are actually observed during interspecies transfer. The experiments will indicate whether there are specific domains on the HA which determine the affinity of the viruses or receptors on cells of different animal species, and whether all or only some of these domains are important in the selection of viruses in nature. They will also indicate whether certain isolation procedures re superior to others when maintenance of the original HA sequence is important, as may be the case during vaccine production and when the virulence and transmission of these viruses are under investigation. Lastly, this study may provide a model for investigating the role of receptor affinity in determining the host range of other enveloped viruses.